Porcelain condenser bushing



Sept- 20, 1960 G. w. LAPP PoRcELAm coNDENsER BusHrNG Filed Nov. 7, 1957s Sheets-sheet 1 Sept. 20, 1960 G. w. LAPP PORCELAIN coNDENsER BUsHING 3Sheets-Sheet 2 Filed Nov. 7, 1957 con/bac r/Vf L 4 vifs w .NNN

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INVENTOR. GROVER W LAPF Sept. 20, 1960 G, w. LAPP 2,953,629

PORCELAIN CONDENSER BUSHING Filed NOV. 7, 1957 5 Sheets-Sheet 3INVENTOR. GROVER Ik LAPP A TTORNEY United States Patent O PORCELAINCONDENSER BUSHING Grover W. Lapp, 68 W. Main St., Le Roy, N.Y.

Filed Nov. 7, 19-57, Ser. No. 695,021

Claims. (Cl. 174-143) This invention relates to electrical insulatorbushings for high voltage use, and more particularly to procelaincondenser bushings such as are used in leading high voltage terminalsfrom oil-immersed switches or transformer windings out through the tankinto the air high enough to prevent flashover.

The usual insulating bushing comprises a center conducting member, aninsulating structure concentric of the conductor, and a metal groundingsleeve by means of which the bushing is maintained in position withrespect to the tank. Where used with oil-immersed electrical equipment,the portion of the bushing, which projects in side the tank orenclosure, is usually either wholly or partially immersed in oil.

In bushings of this type, the internal dielectric stress between thecentral conductor and the grounded tank is distributed by means of equalcapacitances in series. Usually, the capacitances are in the form ofconcentric conductive tubes which are of decreasing length from thecentral conductor outwardly, and the outermost tube is grounded. Theconductor tubes are spaced apart by dielectric layers whose thicknessesvary to obtain a desired constant dielectric relationship. The lengthsof the conductive tubes can be graded in a series of equal steps tocorrespond with the potential gradients of the external path along theupper and lower surfaces of the bushing. The purpose of thisconstruction is to produce condenser layers of equal capacitance todistribute the voltage in approximately equal steps from the centerconducting member carrying the high potential current to the outsidegrounding sleeve at ground potential that supports the bushing on thetransformer cover.

Two basic problems have to be met to achieve a satisfactory condenserbushing. The first is to produce stable dielectric layers between theconductive layers. The second is to make the flashover path resistant tollashover under Wet and dusty atmospheric conditions as well as in dry,clear Weather.

A conventional way of making an insulating bushing has been by windingup paper sheet interlaid with thin conductive foil. In such aconventional bushing, electrical discharges from the sharp edges at theends of the conductive layers of foil at the ends of the foil tend tocreep along the joints between adjacent paper sheets. Because of thetype of construction used there is no opportunity to modify the shape toreduce concentration at the edges of the foil. only fa very low voltageper mil of thickness of the dielectric layer. Moreover, because bushingsof this laminar type are usually immersed in oil, the life of thesebushings is limited since oil vand paper are notoriously susceptible toprogressive deterioration. Other organic insulating materials containcarbon and, when exposed to leakage currents or flashover arcs, tend totrac or establish conductive carbonized paths, and finally to reduce thetiashover voltage.

One object of the invention is to provide a sturdy,

Accordingly, this type of construction can carry simplied condenserbushing having excellent and stable distribution of internal dielectricstress, free from progressive deterioration, and capable of carryinghigh voltage per mil of thickness of the dielectric material.

Another object of the invention is to provide a condenser bushing ofhigh quality, capable of utilization at high eiciency with both low andhigh voltages and without the requirement for oil immersion.

Another object of the present invention is to provide an improvedcondenser bushing of simplified, substantially monolithic, ceramicstructure.

Still another object of the invention is to provide an improvedporcelain condenser bushing having a. self-cooling feature.

Other objects of the invention will become `apparent to those skilled inthe art from the following detailed descn'ption of the invention.

To meet the external problem or making the outside ashover pathresistant to ashover regardless of weather conditions I make my bushingswith an outer shell of fired ceramic material and use corrugations onthis outer shell. Preferably helical corrugations of a drainage type areused to cause rain water to flow in and out over the surface of thebushing while rain IWater is coursing around the bushing in continuoushelical paths. This type and form of corrugations make it possible touse small corrugations having long creepage distance.

To meet the internal problem of obtaining a stable dielectric betweenthe conductive layers of the bushing I embed the conductive layersbetween successive layers of ceramic dielectric.

Two separate embodiments of the invention are illustrated in thedrawings. In one embodiment of the invention, the condenser bushing hasa center conducting member, a novel vitried, substantially monolithicconducting and insulating structure concentrically and symmetricallyarranged around the conductor, and a grounding sleeve adapted to securethe bushing in position on a grounded tank casing. The novel insulatingstructure comprises a plurality of unied layers of clay concentric withthe center conducting member, each layer of clay being separated, forthe major portion of its length, yfrom the overlying layer of clay by aconductive glaze. The concentric cylindrical or tubular layers formed bythe conductive glaze are of different lengths, arranged in a gradedseries of decreasing lengths radially outward from the center conductingmember. Each end of each conductive layer is flared or bulged slightlyoutwardly, then inwardly, to

avoid a high concentration of ilux at the ends of the con-v ductivelayer. The inwardly turned tips ofthe ared ends provide a less pointedemission of grading flux through the vitried clay or porcelain to theexterior of the bushing and the outside ashover path. The outer bushinglayer is of clay forming a sheath around the bushing, except for onesmall spot inside the ilange to bring out the current through a meter,as is usual, for the purpose of measuring capacitance or chargingcurrent and power Ifactor.

At the ends of the bushing, without intervening conductive layers, sothat the vitrified clay with the intervening conductive layers isunified and integrally united, so that an extremely strong structure isobtained in which the conductive layers are permanently enclosed and arecompletely weather-proofed.

To car-ry away lthe heat from the central conductor and to minimizeltemperature differences through the bushing, the central conductor ismade self-cooling. For this purpose it is made tubular, is completelyplugged at both ends, and contains water or other suitable liquid underreduced pressure. Radially-nned heat-dissipating tubes are connected to-this central tubular conductor at its the clay layers overlap upperend4 to radiate therefrom; and these nned tubes are sealed lat theirouter ends, and an arc guard ring is attached around their outer ends. Adrainage channel is rattached below each finned -tube to conduct rainwater collected by this topassembly `and'carry it-down over the bushing;The centralftubular conductor and connected radial nned tubes `forma-tightly; sealedunitwhieh -func-A tions to cool the central conductor.When'` thecentral conductor tube cools to ambient temperature, thevaporpressure inside the tube-falls to correspondto that temperature. If thetemperature-rises the water in the tube boils; vapor rises up in thetube and into the-radial heatdissipating tubes where it is condensed;and-the condensate drips back. to the warmer bottom-of thecentralconductor `tube to be vaporized again. In-this-way, heatisconveyed from the conductor and dissipated into air by the condenser.The larger the condenser the more nearly will the temperature of thetubular central conductor inside the bushing approach the ambienttemperature of the yair above the bushing. With thisstructure thecentral conductor can get rid of its heat, andserve to cool the insidelayers. of dielectric instead of heating them.

When extremely high voltages are to be carried, the bushing is made intwo concentric sections, each having a group of layers of dielectricalternating with conducting glaze. Bach section corresponds in structuregenerally to thenfor-m of bushing already described. The centralconductor isinserted in the bore of the innerl section and is providedwith heat exchangerneans such as described. Between the two sectionsthere is an annularl clearance, and ia concentric double walled tubeislocated in the annular clearance. Waterl or other liquid is` sealed inthis double-walled tube under reduced pressure, andthis tube islinterconnected with external heat exchange means, such as, for example,radially iinned tubes and iagua-rd ring, so that heat will be dissipatedfrom the double-walled tube `and the outer section of the bushing in thesame general manner as from the central conducting tube of the innersection of the bushing.

The `features of the invention will be best understood by considerationof the following description, with reference to the drawings, in which:

Figs. 1 and 1A, taken together, show an axial section of an improvedcondenser bushing made accordingto one embodiment of this invention;

Fig. 2 is a section of apparatus for manufacturing the condenser bushingshown inFig. 1; and

Fig. 3 is an axial section of-a modified form of con-v denser bushingmade accordingtothe invention for carrying high Voltage loads.

Referring now in detail to the drawings, and first to the embodimentshown in Figs. 1 and lA, the condenser bushing comprises 'acentraltubular conductor 11, an insulating assembly 12 arrangedconcentrically around the central conductor but slightly spacedradially-therefrom, `and a short metal grounding sleeve.14 mountedaround the `insulating assembly about midway the length thereof.

The central conductor 11 is an elongated tube having a bore 16 closed inliquid-tight fashion adjacent each end thereof by plugs 18 and 18',respectively. A body of water or Vother liquid 20 is conned within thebore 16 under reduced pressure. The insulating. assembly 12 lis shorterin length than Ithe tubular conductor 11; land the conductor 11protrudes both -at top andV bottom beyond the. insulating assembly. Theinsulating assembly may be spaced slightly radially from the tubularconductor; and

a snug, leak-proof, iianged collar 22 is mounted on topv of ltheinsulating assembly 12 around the central conductor 11', to keep rain`water out of the annular space between.

ring 23 is generally conical in shape and seats on the collar 22,extending at its bottom around the ange of the collar. It surrounds theconductor 11 near the upper end of the conductor and may be made of aconductive material. Each tube 24 has its bore 25 connected at its innerend with the bore of the central conductor and is closed at its otherend. Each tube 24 is provided along its length with radial fins 26. Atrough 2S is mounted -below each tube 24 to provide a drainage channelto conduct rain water which may accumulate on and drip from each finnedtube toward the central conductor 11. An arc guard ring 27 is attachedaround the outer ends of the tubes 24.

The insulating core or assembly 12 comprises a plurality ofconcentrically disposed layers 3), 32, 34, 36 and 38, respectively, ofclay, with which 4are bonded, as a Wholly monolithic s-tructure,interposed concentric conductive layers 40, 42, 44, 46 and 43, formed ascoatings of conductive glaze on the underlying clay layers. Thus, aconductive glaze layer 40 is interposed between the innermosty claylayer 30, `and the adjacent, concentric clay layer 32. A second, shortertubular conductive glaze layer 42 is disposed between the second claylayer 32 and the third concentric clay layer 34, and so forth. Eachsuccessive conductive glaze layer, progressing from the centralconductor 11 outwardly, is of less length than the next inner conductivelayer, so that the lengths are graded in a seriesof preferably equalsteps.

The. clay layers 30, 32, 35, 36 and 38 `are formed to extend, over theentire length of the insulating assembly, `and for purposes'ofillustration only, their boundaries 142 are indicated in Fig. 2. In thevitrified insulating assembly, the several clay layers are fused into a-monolithic whole, s0 that the boundary lines 142 are indistinguishable.

The whole is enclosed in a thick, outer coating of clay 56 havingpreferably helical drainage corrugations 58 on the .outer peripherythereof. The whole insulating assembly is, of course, baked to vitrifyit. The drain channels 28 are disposed to discharge any rain water ontothe upper end or ends of the helical corrugations.

The portion of each clay layer underlying the extremity of each tubularconductive glaze layer is `formed with an outward flare 5), so that theconductive glaze layer is correspondingly contoured, providing Va belledor flared end 52. Each conductive glaze layer terminates at a part ofthe iiared end 52 that is directed inwardly and is formed at its endwith a beadv ring 54 formed in a corresponding groove-in the clay layer.

The metal grounding sleeve 14 is disposed centrally of the bushing 10around the outer layer of vitriiied clay 56. The grounding sleeve 14 issecured by cement 60 or otherl means` on the bushing. An aperture 62 isprovided inthe clay layer 56 inside the ilange of the grounding sleeve14 to permit a plug or probe 145 to be mounted on the flangel to bringout current from the outermost conductive layer 48 through the line 146to a meter 147, and; back to the sleeve 14 through the line 148, as isusual, to measure capacitance or charging current and power factor.

In a typical installation,the ground sleeve 14 is adapted to besupported on and in electrical Contact with a grounded tank casing 64,in which case the lower portion ofv the bushing is adaptedv to besubmerged in oil.

A bottom terminal 66, of metallic conducting material, is mounted on thelower end of the central conductor 11 in electrical contact with it.This terminal may be connected. in conventional manner with theoil-immersed switch or other electrical equipment with which thebushingis to. be used. The portion of the bushing above the groundingsleeve 14 is adapted to be exposed to the atmosphere, and the upperexposed end of the central conductor 11 is adapted. to beconnected withan externall electrical circuit.

One method of manufacturing the condenser bushing of thepresentinvention is described in detail in my copending application,Serial No. 695,058, filed November 7, 1957. For convenience, a briefdescription of a method of manufacture of the bushing is described here.

A typical apparatus for use in the manufacture of the bushing isillustrated in Fig. 2. 4

This apparatus includes a cylinder 110 formed by an elongated tubularpipe 112 interposed between a pair of end plates 114, with a sealinggasket 116 interposed and compressed between each end of the pipe andeach end plate 1214. The end plates are drawn together by any convenientmeans, such as, for example, a plurality of nuts and bolts indicatedgenerally by the numeral 118.

An elongated, completely threaded bolt 120 passes through apertures 122in the end plates and centrally through the cylinder 110. A mandrel 124is mounted around the elongated bolt 120 Within the cylinder 110. Themandrel 124 is preferably sectional and is made from a porous orWater-permeable mixture of Portland cement and sand or `other filler.The mandrel surface 126 is preferably slightly tapered from one end tothe other to facilitate its removal from the completed blank.

The mandrel has an open bore 128 through which the threaded bolt 120passes freely. At each end of the mandrel a metallic end bushing 130 ismounted. Each end bushing has an inner end face 132 coinciding in sizewith the diameter of the mandrel at that end `and engaging the adjacentend `of the mandrel, from which the end bushing tapers gradually outwardto coincide approximately with the outer diameter of the condenserbushing at that end. A gasket 136 is interposed between the opposedsurfaces of each end bushing 130 and each end plate 114.

In manufacturing a condenser bushing with this apparatus, the mandrel126 is mounted on the elongated bolt 120, together with the end bushings130. Wet plastic clay is built up in a single layer around the mandrelin excess thickness to form the first dielectric layer 30. This may bedone by casting from clay slip, or by plaster" ing the clay on, or bylaying it on by an injection-plugging method in my copending applicationreferred to above.

It is preferred to lay a perfectly air-free layer, but not necessarysince any inclusions of air in the clay will be removed in the next stepin the process. A rubber sheath, similar to sheath 138, is placed over,or a rubber ribbon is wound around the layer of wet plastic clay. Careis taken that the sheath or ribbon extends around the end bushings 130.The sheathed cast is then placed in the cylinder 110. Hydraulic pressureon the order of 1000 p.s.i. is then applied to the sheath. This pressureforces the sheath tightly against the layer of clay, to cause the clayto be dewatered, compacted, and densied. Water forced out of the clay,passes through the porous mandrel 126, and escapes in the void spacebetween the threads of the bolt 120 and the bore 128 of the mandrel. Thepressure is maintained until water stops dripping, or

until the clay is considered sufficiently dewatered andV densiied. Ifdesired, the pressure may be progressively increased during this step. Y

When dewatering is completed, the pressure is released and the mandreland its clay coating are removed from the cylinder 110. The rubbersheath or rubber ribbon is then removed and the layer of clay is turnedin a lathe, preferably in a vertical position to avoid any tendency tosag. The consolidated or compacted clay is -then in excel lent conditionto be precisely turned to size, leaving the portion that is to be givena conductive coating slightly smaller than the ends. The ends of theprecisely sized section are ilared slightly to give a graduallythickened wall, and the outer tips of the flarred ends can be turnedinwardly-slightly. This is for the purpose of shaping the clay so thatwhen the conductive layer is deposited on the compacted clty layer,concentrated ilux at the ends of the conductive layer will be avoided,and there will be less pointed emission ofthe grading ux through theporcelain to the outside ashover path of the nished bushing. The

clay is turned precisely to the particular length, diameter, and aredend shape desired.

After the clay layer has been machined to the precise shape desired, theends of the clay layer, which are beyond the contour portion can betaped to mask them; and a conductive glaze 40 of metallic oxides is thenapplied, having ared ends 50 and small beads 54 in the end grooves ofthe clay layer 30. The glaze must be refractory so that it will not fuseto a pre-vitried layer. It must remain permeable to the exit of gasesand must be free of constituents such as copper, that tend to migrateinto the clay during ring.

After the conductive glaze coating has set so that it will not Wash, asecond clay layer 32 is applied. The clay layer is then wrapped again inthe rubber sheath 138 or in a rubber ribbon, which is extended aroundthe end bushings 130, and the assembly is placed in the cylinder andpressure is again applied. The cast is thus dewatered by hydraulicpressure. Then the whole is removed from the pressure tank 110, andunsheathed,

and the newly-applied clay layer is turned to the thick-A ness andcontour desired, and another conductive coating 42 is applied, which isshorter and of larger diameter than coating 40.

This sequence of operations is repeated until the nal conductive glazecoating 48 has been applied. In this way, successive layers ofdielectric and conductive coat` ings are built up. The iinal or outsidecover layer of porcelain body is then applied as a final thick coatingof clay around the conductive coating 48, one small spot being leftinside the flange 23 to bring out the current `from the ground sleeve 11through a meter for the purpose of measuring capacitance, or chargingcurrent and power factor. The outermost layer may be wound in a porouslayer of glass Wool or tape 137, which in turn is covered by the rubbersheath 138. A pipe or metallic tubing 140, that extends through the endplate 114, is provided to drain oif water from the pressure tank.

During successive dewatering steps, the pressure applied to the sheath138 is programmed so that dewatering of the iinal thick outer layer 56is accomplished at the highest pressure. This has the desirable resultthat the successive clay layers 30, 32, 34, etc. and the intermediateglazed coatings 40, 42 44, etc. are compacted about the mandrel, andformed as a substantially monolithic whole. At lthe ends of thesuccessive clay layers, where conductive glaze is not interposed betweenthem, their boundaries, indicated in the drawing by the dotted lines142, are not distinguishable.

When the dewatering and densication steps on the outer layer 56 havebeen completed, the rubber sheath and the porous layer are removed; andthe insulating assembly blank is placed in a lathe and turned to formthe helicoidal corrugations 58 in the outermost clay layer 56. Ifconvenient, the blank may be left on the porous mandrel 126 whileturning on the lathe.

After the mandrel is removed, the clay blank is reamed to a diameterapproximately 5% larger than the desired diameter. The bore of the blankis then coated with a conductive glaze, and the blank is tired. Duringring, plastic ilow completely fuses adjacent clay layers so that ahomogeneous, unitary, and monolithic bushing is thus obtained. Aftertiring, the remaining components of the bushing are assembled on thetired core.

This construction provides conductive contoured layers embedded in solidvitriiied porcelain, hermetically sealed away from the weather andatmospheric moisture. Such a bushing may be housed in a porcelain,oil-filled housing as usual and will give excellent performance.However, because of the presence of the outer thick corrugated layer ofporcelain, the bushing may also be employed as a dry type of bushingwithout oil. Because the bushing is suitable for use as a dry bushing,even at high voltages, troublesome design problems, which wouldotherwise be encountered, are avoided.

Simplicity of structure is a markedV characteristic of this type ofbushing. Because of the stability ofY fused ceramic asl compared withorganicvr dielectric material, the structure will have long life withoutincrease in dielectric power factor. service can, therefore, be avoided.

The flared end contour` of each conductivel layer eliminates hot spots,or points of critical dielectric stress.

As a result ofthe described method of manufacturing my bushing, wheneach layer` of clay is. dewateredunder known pressure, and then aconductive layer is applied, followed by another pressure-dewatered`layer of clay, there is thorough consolidation ofthe ceramic granules,nodrying shrinkage, andthe-final firing shrinkage isvery much reduced.The residual firing shrinkage can be controlled by programming thesuccessive` pressures and the finalconsolidating pressure to achieve afinal solid structure when fired. By eliminating: all drying shrinkageandav large part of the firingv shrinkage, the residual final shrinkageoccurs in the range of early vitrication when the body is hot enough tobe safer against cracking because it is somewhat plastic. Since thesuccessive layers are thoroughly compacted, differential shrinkages arelargely eliminated and warping is minimized. thisway, roundness andstraightnessof` the tubular forms are improved, and dimensional controlis made more exactly predictable.

When the condenser bushing becomes heated in operation, the liquid,which is sealed in the bore of` the central conductor, boils under thereduced pressure obtaining. Vapor rises up and condenses in the radiallyiinnedtubes 24, and the condensate returns to the bottom of theconductor 11, where it, again takes on heat. in this way, heat istransferred from the interior of the bushing through the heat exchangetubes 24 to the atmosphere. Moreover, the radially finned heat exchangetubes 24 serve to dissipate electrostatic flux to help grade theflashover path.

For simplicity of illustration and description, the fins 26 of the tubes24 have been illustrated and described as of uniform size. In order toincrease the clearance distance between the surrounding arc guard rings,the finned tubes can be made shorter on the near sides. and longerV onthe outside. Also, other sets of finned tubes can be placed above therst set of tubes.

For bushings of higher voltage ratings, the thickness of wall may exceedthe practical thickness. for firing ceramic bodies. In this case, thebushing may be manufactured in two or more sections, as illustratedinFig. 3. This practice is advisable also because of differences intemperature that may exist between the outside and inside layers due-toheating and cooling by atmospheric changes on the outside, or due to thesuns heat, or duetto heating by dielectric losses and by resistancelosses in the central conductor which carries the load current.Differential heating and cooling cause expansion and contraction,resulting in strains in the structure.

The high capacity bushing illustrated in Fig. 3 includes a centraltubular conducting member 111 and a first or inner insulating assembly,119 and a second, or outer insulating assembly 1349. In each assembly,layers of conductive glaze are interposed between layers of clay, as inthe first describedy embodiment of my invention. The central tubularconducting member is cooledy as in thel first-described embodiment of myinvention by radially finned heat exchange tubes 123. In addition, inthe bushing of Fig. 3, a sealed, double walled tube 132 is locatedy inthe annular space between the two sections of the bushing, The tube isclosed and contains water or other liquid underV reduced pressure. Aplurality of heat exchange tubes 125 are secured around the bushing tothe double walled tube 132, and the bore of each interconnects at itsinner end with the bore of the tube, while the outer end ofT eachV isyclosed. The karray of heat dissipating tubes serves, not only for Muchtesting, and replacement inv InA coolingthe bushing but also todissipate electrostatic flux to help grade-the flashover path.Additional sets of finned tubes can obviously be mounted above the setsillustrated, if desired, to increase the dissipating capacity.

The bore 121 in the inner assembly 119 of the insulating assembly iscoated with a conductive glaze (not shown) to avoid corona from thecentral conductor 111 to the inside` wall. Similarly, the exteriorsurface 133 of the inner section 119, which is juxtaposed with thedouble walled tube 132, and the opposed inner surface 135 of the outersection 130, are coated with conductive glaze (not shown).

A collar 134 is mounted on top of the inner section 119, toV preventseepage of rain water into the space 121. Appropriate gaskets, notshown, are provided between the top of the porcelain of the bushing andthe top conductor flange, and also between the inner assembly 119 andthe outer assembly where exposed to the weather.

A grounding sleeve 114 is mounted on a smooth portion of the outersection 130 of the insulating assembly.

The operation of this multi-sectional condenser bushing is similar tothat of the bushing previously described. The heat exchange meansprovided between the two sections eliminates any difference intemperature that may exist, between the two sections and thuspreventsdifferential expansion or contraction which would result instrains in the-structure.

Eachsection is separately made by the general process previouslydescribed, and the sections and other cornponents of the bushing are'assembled to complete the final structure shown.

While the invention has been described in connection with specificembodiments thereof, then, it will be understood that itis capable offurther modification, and this application is intended to cover anyvariations, uses, or adaptations of the invention following, in general,the principles of the invention and including such departures from thepresent disclosure as come within known or customary practice in theart4 to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as fall within the scopeof the invention or the limits of the appended claims.

Having thus described my invention, what I claim is:

1. An electricalinsulator bushing having a central bore and comprisingalternating layers of ceramic insulating and of electrically conductivematerial, said insulating layers extending at their ends beyond theconductive layers, said conductive layers comprising conductive filmsdcposited on the respectively subjacent insulating layers and arrangedconcentrically of each other in a graded series in which the films areof progressively decreasing lengths outwardly from said bore, thecorresponding ends of successive films being displaced progressivelyaxially of said bore, each film ycomprising a layer of metallic oxide,each end of each film being convex and being directed radially inwardlyof said member, said films being mutually spaced and electricallyinsulated from each other by said interposed layers of insulatingmaterial, and an outer layer of ceramic insulating material surroundingand encasing said alternating layers, the extended end portions of saidceramic insulating layers being integrally united together and to theouter layer-of ceramic insulating material to form a substantiallymonolithic structure, the exterior surface of said member comprisingalternating lands and grooves extending around the periphery of saidmember.

2. An electrical insulator bushing comprising a central conductor rodhaving a bore sealed Aat each end thereof, heat exchange fluid confinedwithin said bore, radially finned tubes mounted radially around said rodadjacent the top of said bushing and centrally bored with the boresthereof interconnecting with the bore of said rod, an insulating andcondenser assembly concentrically mounted on said rod, and a groundingsleeve mounted concentrically around thel insulating and condenserassembly, said insulating and condenser assembly comprising alternatinglayers of ceramic insulating and of electrically conductive material,said insulating layers extending at their ends beyond said conductivelayers and being integrally united at their ends to form a substantiallymonolithic structure, said conductive layers cornprising a series ofconcentric, substantially tubular conductive members and being arrangedaccording to a decreasing progression of length outwardly from said rod,said conductive members being mutually spaced and electrically insulatedfrom each other by the said interposed layers of ceramic insulatingmaterial.

3. An electrical insulator bushing comprising a central conductor rod,an insulating and condenser assembly concentrically mounted on saidconductor rod, and a grounding sleeve mounted concentrioally around saidinsulating and condenser assembly, said insulating and condenserassembly comprising at least two concentric sections, each sectionhaving a central bore and comprising alternating layers of insulatingand electrically conductive material, the insulating layers extending attheir ends beyond the conductive layers and being integrally united attheir ends to form a substantially monolithic structure, said conductivelayers comprising a series of substantially tubular conductive membersconcentrically disposed relative to said central conductor rod andhaving areas which are arranged in a decreasing progression outwardlyfrom said rod, said conductive members being mutually spaced andelectrically insulated from each other by the interposed layers ofinsulating material.

4. An electrical bushing comprising a central con* ductor tube, aninsulating and condenser assembly concentrically mounted on said tube,and a grounding sleeve concentrically mounted around said insulating andcondenser assembly, said insulating and condenser assembly comprising atleast two concentric sections, each section having a central bore andcomprising alternating layers of insulating and electrically conductivematerial, a closed chamber disposed in the annular space between eachpair of concentric sections, liquid under reduced pressure confined ineach chamber and in said central conductor tube, finned tubescommunicating at their inner ends with said central conductor tube andsecured to said tube and mounted above said insulating and condenserassembly and being closed at their outer ends, and other finned tubescommunicating with each closed chamber between concentric sections andalso being closed at their outer ends.

5. An electrical bushing adapted for use in )a high voltage circuit andcomprising a central conductor tube through which courrent is adapted topass, an insulating and condenser assembly concentric of said tube, anda grounding sleeve mounted concentrically around said insulating `andcondenser assembly and adapted to be at ground potential, saidinsulating and condenser assembly including at least two concentricsections, each section having a bore, said conductor tube extendingthrough the bore of the inner section, and each section comprisingalternating layers of insulating and conductive ma* terial, theinsulating layers extending at their ends beyond the conductive layers,said conductive layers comprising substantially tubular electricallyconductive iilms arranged concentrically of said tube in a series inwhich the iilms are of progressively decreasing areas outwardly fromsaid tube, each end of each iilm being displaced from the correspondingend of the next subjacent lm axially of the bushing toward saidgrounding sleeve and each end of each film being flared outwardlythrough a convex path and terminating along a slope of said pathdirected radially inward toward said conductor tube, each iilmcomprising a metallic oxide, said iilms being mutually spacedandelectrically insulated by the interposed layers of insulatingmaterial, an outer layer of insulating material encasing saidyalternating layers, the extended ends of said alternating layers ofinsulating material being united together and united with the outerlayer of insulating material to form a monolithic structure for eachsection, said outer layer of each section being helically corrugatedover at least a portion of its length, heat exchange means for saidconductor tube, and heat exchange means interposed between each pair ofconcentric sections of said insulating and conductive assembly, theoutermost of said conductive ilrns in the outer section beingelectrically connected to said grounding sleeve.

References Cited in the le of this patent UNITED STATES PATENTS

